Activated carbon is a material used extensively for water treatment, food processing, air purification, energy storage, and vehicle fuel recovery. In 2013, the United States used about 480 million pounds of activated carbon for these applications. Activated carbon is made from coal or biochar through activation, which is a critical step to creating porous nanostructures in carbon materials having a large surface area, proper distribution of pore size, and high surface energy.
Previous activation methods require high temperatures and are inefficient. For example, coal-based steam activation is conducted at high temperatures (>700° C.) and the yield is only about 45%.
In contrast to limited coal resources, biochar obtained from biomass pyrolysis is a “green” and sustainable material that is expected to eventually dominate the market of activated carbon. Unfortunately, conventional thermal activation of biochar also needs high temperatures (700-1200° C.) for hours using steam, CO2, and/or a strong base (e.g. KOH) followed by chemical washing (to remove the residual base) and prolonged drying, respectively. This energy-intensive and lengthy treatment has become a critical barrier to meeting the globally increasing demands for activated carbon.
Furthermore, in the traditional thermochemical activation with convective and/or conductive heating, the biochar temperature is generally not uniform, depending on shapes and sizes of the material. This non-uniform heating causes local overheating and leads to low yield due to the complete combustion of part of the carbon.
Therefore, there is a strong need in industry for more efficient and effective methods of activating biochar.